Diesel Generator Sizing – What Size Does Your Business Need?

What Size Diesel Generator?

The whole point of a back-up diesel generator is to provide all the electrical power you need to keep your business or organisation running at full speed...

...even if the local power grid proves unreliable. This may or may not mean powering your whole facility, you may just be concerned about the services covered by one distribution board. Obviously, you need the generator’s capacity to at least equal the peak consumption of the equipment that you need to keep online. However, simply buying the biggest generator you can afford (or accommodate) can be both expensive and inefficient.

Diesel Generator Size Guide

This guide will teach you the basics of picking a generator that is just big enough to get the job done, with enough of a safety margin that your entire operation doesn’t grind to a halt – again – just because someone plugged in a toaster.

1) Have a certified electrician inspect your premises, and determine your maximum power load.
Alas, this is the step most people skip, even though it is probably the most important. Most of the next steps outline how to do this for yourself, but you really are better off having a professional make the calculations based on knowledge and experience.

2) Determine your power requirements.
First, make a list of all of the items that need to run on back-up power. This should include not just your key pieces of equipment, but also lighting, computers and desktop appliances, and everything that you actually need to get any real work done. Now note the starting wattage and running wattage for each
Most of these values will be listed, either on an information plate that should be affixed to every electrical device, or in the device’s owner’s manual. If you no longer have the owner’s manuals, many manufacturers post free .pdfs of their manuals online.
Convert all these values into the same units – usually either kW or kVA. You can convert Amperes (‘Amps’) to Wattage fairly easily. For resistive loads, you simply multiply the Amperage by the Voltage to get the Wattage. For reactive loads, use Amps multiplied by Volts and then multiply the result by the load factor to get the wattage. If none of that made sense at all, you really do need an electrician or some other expert.
Simply add the largest column of numbers together, and you have your peak power consumption.

3) Select a diesel generator or other back-up genset with a rating at least 5%-10% higher than that power requirement.
Alas, this is the step most people skip, even though it is probably the most important. Most of the next steps outline how to do this for yourself, but you really are better off having a professional make the calculations based on knowledge and experience.
A margin for error, and to account for unexpected extra loads, is vital. You really shouldn’t be running your genset at its flat-out maximum capacity for long periods. That would be like driving with the accelerator all the way down – most cars can’t handle it for long, and even a top-end genset will wear out faster if constantly redlined.

Choosing a Diesel Generator with slightly more capacity than you need gives you the following benefits:

The generator won’t shut down when you need it most due to being over loaded.

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